STRUCTURAL STUDY OF ELLENBERGERITE .2. EFFECTS OF HIGH-PRESSURE

Compressibility and structural deformations induced by pressure in the high-pressure silicate ellenbergerite [Mg,(Ti,Zr),square]2Mg6(Al,Mg)6 (Si,P)8O28(OH)10; hexagonal, space group P6(3), have been studied by s ingle-crystal X-ray diffraction in a diamond anvil cell (DAC) in the p ressure range 1 bar - 46 kbar. The bulk modulus of the mineral is 1330 kbar. The compressibility pattern is slightly anisotropic, with beta( a) = 2.4 10(-4) and beta(c) = 3.1 10(-4) kbar-1. Compressibility is co nstant in the pressure range examined. Limited compositional differenc es of the samples do not produce significant variations in the compres sibility coefficients. The geometrical data vs. pressure fit along the linear trends: a = a0 - 0.0029 P c = c0 - 0.0015 P with P in kbar and a and c in angstrom. In agreement with its occurrence in coesite-bear ing rocks, ellenbergerite is a very dense phase, with a structure form ed by chains of face-sharing octahedra, which represent rigid structur al moduli. Therefore the structural response to the high pressure appl ied in the DAC consists of small deformations in octahedral bond lengt hs and angles, with only minor interpolyhedral geometrical variations. Comparison between high-pressure and high-temperature data indicates that the unit cell and bond geometry of ellenbergerite at formation co nditions, namely 700-800-degrees-C and 25-30 kbar, do not differ great ly from those at room conditions.